This invention relates to in situ recovery of shale oil and more particularly to a technique for forming an in situ oil shale retort in a subterranean formation, the retort containing a fragmented permeable mass of formation particles having a reasonably uniformly distributed void fraction.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. The term "oil shale" as used in the industry is, in fact, a misnomer, it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit containing an organic material called kerogen which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the carbonaceous liquid product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the oil shale and processing it on the ground surface or processing the oil shale in situ. The latter approach is preferable from the standpoint of environmental impact since the treated oil shale remains in place reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,597; and 4,043,598, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous products from a subterranean formation containing oil shale wherein such formation is explosively expanded for forming a fragmented permeable mass of formation particles containing oil shale within the subterranean formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the fragmented mass in the retort for converting kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen-supplying inlet mixture for advancing the combustion zone through the fragmented mass. In the combustion zone oxygen in the mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and spent oil shale. By the continued introduction of such a mixture into the retort the combustion zone is advanced through the fragmented mass.
Hot effluent gas from the combustion zone passes through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces liquid and gaseous products and a residual solid carbonaceous material in the retorted oil shale.
The liquid and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. Shale oil, together with water produced or added to the retort collects at the bottom of the retort and is withdrawn. An off gas is also withdrawn from the bottom of the retort, including gaseous products of retorting, combustion products and any portion of the inlet mixture that does not take part in the combustion process.
A fragmented mass of particles can be formed in an in situ oil shale retort by excavating a void or voids within the retort site. Formation remaining within the retort site is then explosively expanded toward such voids for forming a fragmented permeable mass of formation particles. In effect, the volume of the voids excavated in the retort site is distributed between the particles of fragmented formation upon explosive expansion. The void fraction, typically expressed as a percentage, is the volume of the fragmented mass occupied by such void spaces between the particles. Thus, for example, in a fragmented mass having an average void fraction of 25%, the void spaces occupy 25% of the total volume of the fragmented mass and the particles occupy 75% of the total volume.
The arrangement of voids excavated in a retort site and techniques used for explosively expanding formation towards the voids can affect the uniformity of distribution of void fraction and hence permeability within the fragmented mass. It is desirable to have reasonably uniformly distributed void fraction in the fragmented mass for uniformity in permeability. This is desirable so that retorting gas can flow generally uniformly through the fragmented mass during retorting operations.
It is also desirable to have a reasonably high void fraction throughout the fragmented mass so that the total pressure drop due to gas flowing through the fragmented mass is not excessive. Gas may be passed through a retort for several months and an excessive pressure drop can result in considerable energy expenditure for causing the gas flow. Excessive pressure drop can also require use of gas blowers that are more costly than lower pressure fans.
It is therefore desirable that techniques used for forming an in situ oil shale retort provide means for controlling the void fraction distribution within the fragmented mass. Techniques in accordance with practice of this invention facilitate control over the void fraction distribution in a fragmented mass being formed.